The Semantic Web will be composed of a series of standards. These standards have to be organised into a certain structure that is an expression
of their interrelationships. A suitable structure is a
hierarchical, layered structure.
The slide illustrates the different parts of the
Semantic Web in the vision of Tim Berners-Lee. The
notions are explained in an elementary manner here.
Layer 1. Unicode and URI
At the bottom there is Unicode and URI. Unicode
is the Universal code. Unicode codes the characters of
all the major languages in use today.
Layer 2. XML, namespaces and XML Schema
Layer 3. RDF en RDF Schema
The first two layers consist of basic internet
technologies. With layer 3 the Semantic Web begins.
RDF Schema (rdfs) has as a purpose the
introduction of some basic ontological notions. An
example is the definition of the notion “Class” and “subClassOf”.
Layer 4. The ontology layer
The definitions of rdfs are not sufficient. A
more extensive ontological vocabulary is needed. This
is the task of the Web Ontology workgroup of the W3C
who has defined already OWL (Ontology Web Language) and OWL Lite (a subset of OWL).
Layer 5. The logic layer
In the case study the use of rules was
mentioned. For expressing rules a logic layer is
needed. An experimental logic layer exists.
Layer 6. The proof layer
In the vision of Tim Berners-Lee the production
of proofs is not part of the Semantic Web. The reason
is that the production of proofs is still a very active area of research and it is by no means possible to make a
standardisation of this. A Semantic Web engine should only need to verify proofs. Someone
sends to site A a proof that he is authorised to use
the site. Then site A must be able to verify that
proof. This is done by a suitable inference engine. Three inference engines that use the rules that can be defined with this
layer are: CWM, Euler and RDFEngine developed as part
of this thesis.
Layer 7. The trust layer
Without trust the Semantic Web is unthinkable.
If company B sends information to company A but there
is no way that A can be sure that this information
really comes from B or that B can be trusted then there remains nothing else to do but throw away that information. The same is
valid for exchange between citizens. The trust has to
be provided by a web of trust that is based on
cryptographic principles. The cryptography is necessary so that everybody can be sure that his communication partners are who they
claim to be and what they send really originates from
them.
The trust policy is laid down in a “facts and
rules” database. This database is used by an inference
engine like RDFEngine. A user defines his policy using a GUI that produces a policy database. A policy rule might be e.g. if the
virus checker says ‘OK’ and the format is .exe and it is signed by ‘TrustWorthy’ then
accept this input.
The impression might be created by the slide
that this whole layered building has as purpose to
implement trust on the internet. Indeed it is necessary for implementing trust but, once the pyramid of the slide comes into
existence, on top of it all kind of applications can
be built.
Layer 8. The applications
This layer is not in the figure; it is the
application layer that makes use of the technologies
of the underlying 7 layers. An example might be two companies A and B exchanging information where A is placing an order with
B.